We investigate the magnitude of the perturbation in a comet's energy caused by nongravitational forces and how these forces affect the dynamical evolution of long-period (LP) comets. From the study of the distribution of original energies or reciprocal semimajor axes ( 1/a) orig of the observed sample of near-parabolic comets, we conclude that the nongravitational perturbation in a comet's energy per orbital revolution is generally smaller than ∼ 10 -4 A.U. -1. If this result were not the case, the Oort peak (i.e. the concentration of comets in the range 0 < ( 1/a) orig < 10 -4A.U. -1) would be erased or severely weakened, which is only observed for comets with q ≲ 0.25 A.U. The Yeomans-Chodas model of asymmetric nongravitational forces is used to model the perturbation in 1/a per perihelion passage for a sample of hypothetical parabolic comets. Our results fit the observations reasonably well if a nongravitational acceleration magnitude of ∼ 10 -8 A.U. day -2 is adopted. This acceleration agrees with estimates for nongravitational forces acting on a few observed LP comets as presented in the Marsden and Williams "Catalogue of Cometary Orbits" (Smithsonian Astrophysical Observatory, 1992). The nongravitational acceleration of LP comets is found to be on average one to two orders of magnitude greater than that for short-period (SP) comets. This result is interpreted in terms of a greater activity (i.e. a larger fraction of active surface) on LP comets. Nongravitational perturbations dominate over planetary perturbations only for LP comets approaching the Sun to distances ≲0.1 A.U. Therefore, nongravitational forces may play only a minor role in the evolution of a comet's energy, except perhaps for "sungrazers" or those LP comets suffering splittings or violent outbursts.